This invention pertains to Raman amplifiers and, more particularly, to Raman amplifiers having a bandwidth which exceeds the peak Raman Stokes gain shift of the transmission medium with which the Raman amplifier is utilized.
Optical fiber technology is currently utilized in communications systems to transfer information, e.g., voice signals and data signals, over long distances as optical signals. Over such long distances, however, the strength and quality of a transmitted optical signal diminishes. Accordingly, techniques have been developed to regenerate or amplify optical signals as they propagate along an optical fiber.
One well-known amplifying technique exploits an effect called Raman scattering to amplify an incoming information-bearing optical signal (referred to herein as a xe2x80x9csignal wavelengthxe2x80x9d). Raman scattering describes the interaction of light with molecular vibrations of the material through which the light propagates (referred to herein as the xe2x80x9ctransmission mediumxe2x80x9d). Incident light scattered by molecules experiences a downshift in frequency from the power-bearing optical signal (referred to herein as the xe2x80x9cpump wavelengthxe2x80x9d). This downshift in frequency (or increase in wavelength) from the pump wavelength is referred to as the xe2x80x9cStokes Shift.xe2x80x9d The downshift of the peak gain from the pump wavelength is referred to herein as the xe2x80x9cpeak Stokes shift.xe2x80x9d The extent of the downshift and the shape of the Raman gain curve is determined by the molecular-vibrational frequency modes of the transmission medium. In amorphous materials, such as silica, molecular-vibrational frequencies spread into bands which overlap and provide a broad bandwidth gain curve. For example, in silica fibers, the gain curve extends over a bandwidth of about 300 nm from the pump wavelength and has a peak Stokes shift of about 100 nm.
The overall concept of Raman scattering is well known and is described in numerous patents and publications, for example, R. M. Stolen, E. P. Ippen, and A. R. Tynes, xe2x80x9cRaman Oscillation in Glass Optical Waveguides,xe2x80x9d Appl. Phys. Lett, 1972 v. 20, 2 PP62-64; and R. M. Stolen, E. P. Ippen, xe2x80x9cRaman Gain in Glass Optical Waveguides,xe2x80x9d Appl. Phys. Lett, 1973 v. 23, 6 pp. 276-278), both of which are incorporated herein by reference. With respect to the present invention, the most relevant aspect of Raman scattering is its effect on signal wavelengths traveling along the transmission medium.
FIG. 1 illustrates prior art optical amplifier which utilizes Raman scattering to amplify a signal wavelength. Referring to FIG. 1, a pump wavelength xcfx89p and a signal wavelength xcfx89s are co-injected in opposite directions into a Raman-active transmission medium 10 (e.g., fused silicon). Co-propagating pumps may be used, although a counter-propagation pump scheme reduces polarization sensitivity and cross talk between wavelength division multiplexed (WDM) channels. Providing the wavelength of the signal wavelength xcfx89s is within the Raman gain of power wavelength xcfx89p (e.g., about 300 nm in silica), the signal wavelength xcfx89s will experience optical gain generated by, and at the expense of, the pump wavelength xcfx89p. In other words, the pump wavelength xcfx89p amplifies the signal wavelength xcfx89s and, in so doing, it is diminished in strength. This gain process is called stimulated Raman scattering (SRS) and is a well-known technique for amplifying an optical signal. The two wavelengths xcfx89p and xcfx89s are referred to as being xe2x80x9cSRS coupledxe2x80x9d to each other. A filter 16 transmits all signals of the signal wavelength xcfx89s and blocks signals of the pump wavelength cop thereby filtering out the pump wavelength.
FIG. 1A illustrates the gain curve for a signal wavelength xcfx89s amplified using a single pump wavelength xcfx89p. As shown in FIG. 1A, while gain occurs over a broad bandwidth (e.g. 300 nm in silica), only a portion of it (e.g., about 50 nm) is, from a practical standpoint, useable to effectively amplify the signal wavelength xcfx89s. This useable bandwidth is referred to herein as thexe2x80x9ceffective Raman gain.xe2x80x9d The effective Raman gain is determinable by one skilled in the art and depends on a number of factors including the desired degree of amplification and the desired flatness across the amplification bandwidth. In silica, the effective Raman gain having less than 3 dB gain variation extends about 25 nm on either side of the peak Raman Stokes shift of about 100 nm. Therefore, the bandwidth of the effective Raman gain occurs from about 75 to about 125 nm from the pump wavelength as shown between points A and B on the Raman gain curve in FIG. 1A.
FIG. 2 is a schematic drawing illustrating the relationship between pump wavelengths and the signal wavelengths of a prior art Raman amplifier. The schematic of FIG. 2 shows multiple pump wavelengths cop through xcfx89p+n which are used to amplify signal wavelengths xcfx89s through xcfx89s+m. Because the effective Raman gain occurs about 75to about 125 nm from the pump signal, signal wavelengths separated from a pump wavelength within this range will be effectively SRS coupled to the pump wavelength. In FIG. 2, pump wavelength xcfx89p (1370 nm) is separated from signal wavelength xcfx89s (1470 nm) by approximately 100 nm. Thus, assuming that the transmission medium 10 of FIG. 1 is silica, pump wavelength xcfx89p will be SRS coupled to and amplify signal wavelength xcfx89s.
If only a single pump wavelength xcfx89p is used, only signals in the bandwidth from xcfx89sxe2x88x9225 nm to xcfx89s+25 nm would be within the effective Raman gain. However, the use of multiple pump wavelengths xcfx89p through xcfx89p+n as shown in FIG. 2 allows the gain bandwidth to be expanded to amplify signal wavelengths xcfx89s through xcfx89s+m. Furthermore, the use of multiple pump wavelengths serves to reduce gain variation (improve flatness) within this bandwidth due to the cumulative effect of multiple gain curves.
Despite multiple pump configurations, prior art Raman amplifiers are nevertheless limited in bandwidth, which in turn limits the capacity of WDM systems. More specifically, because the effective Raman gain tails off at about 125 nm from the pump wavelength, signal wavelengths beyond this point are not effectively amplified. Furthermore, the applicants have found that in multi-pump systems, where excellent flatness in amplification is achievable through the cumulative effect of multiple gain curves, signal wavelengths preferably should be within the peak Stokes shift of a pump wavelength, e.g., about 100 nm, for optimum flatness. This limitation in SRS coupling limits the bandwidth of signals, e.g. xcfx89s from xcfx89s+m as shown in FIG. 2, to the peak Stokes shift of a pump wavelength since extending the signal bandwidth beyond xcfx89s+m would require introducing pump wavelengths into the signal bandwidth, beyond xcfx89s.
Injecting pump wavelengths into the signal bandwidth, however, has traditionally been avoided due to backward Rayleigh scattering (BRS) resulting from the pump signals. BRS results from random localized variations of the molecular positions in glass that create random inhomogeneities of the reflective index that act as tiny scatter centers. Although the pump and signal wavelengths can be easily separated by filtering in a counter-propagating scheme, the BRS from the pump wavelengths, which propagates in the direction of the signals, is not easily filtered. Furthermore, BRS from longer pump wavelengths falls into the Raman gain generated by shorter pump wavelengths, thereby causing this BRS to be amplified such that it equals or exceeds the intensity of the signal wavelengths. For example, a pump wavelength generated at point A and intended to amplify a signal wavelength at point B would coincide with signal wavelength xcfx89s+2. The BRS from the pump wavelength at point A is affected by the Raman gain of the lower pump wavelengths, thus introducing undesired noise into the signal wavelengths near point A (FIG. 2). Thus, BRS both decreases the Raman amplification of the adjacent signals by depleting the pump wavelengths"" power, and diminishes signal quality by introducing noise and cross-talk between the channels.
BRS also causes a four-wave-mixing effect. Four-wave-mixing is defined by third order susceptibility in the relation between the induced polarization from the electric dipoles and electric field. In a particular case of four-wave-mixing in optical fibers, a strong pump wave at a frequency xcfx891 creates two side bands located symmetrically at the frequencies xcfx892 and xcfx893. The frequency shift of the side bands is given by xcexa9s=xcfx891xe2x88x92xcfx892=xcfx893xe2x88x92xcfx891, where xcfx892 less than xcfx893. The phase matching requirement for this process is k2xe2x88x92k3xe2x88x922k1=0, where k is the wave number. The two side bands may also introduce undesired noise into the signal wavelengths.
Accordingly, it would be desirable to have a method and apparatus for expanding the gain bandwidth of a Raman amplifier beyond the maximum gain Raman Stokes Shift of the transmission medium without the attendant problems of BRS.
In accordance with the present invention, the amplification bandwidth of a Raman amplifier is expanded by interleaving narrow pump wavelengths between signal wavelengths, thereby avoiding interaction between the signal wavelengths and the BRS of the interleaved pump wavelengths. The line width of the pump signals are narrow enough (e.g., less than 1 GHz) compared to the wavelength spacing of the signal wavelengths (e.g., as low as 25 GHz) so that the BRS of the interleaved pump wavelengths is readily distinguishable from signal wavelengths and can be efficiently filtered out.
One aspect of the present invention is a method of Raman amplification employing interleaved pump and signal wavelengths. In a preferred embodiment, the method comprises effecting a plurality of pump wavelengths on a Raman-active transmission medium which is transmitting counter-propagating signal wavelengths, wherein one or more of the pump wavelengths are interleaved between the signal wavelengths. Preferably, the plurality of pump wavelengths spans a bandwidth that exceeds the peak Raman Stokes Shift of the transmission medium. In a preferred embodiment, the method further comprises the step of reducing BRS generated from the interleaved pump wavelengths.
Another aspect of the present invention is a Raman amplification system for amplifying signal wavelengths propagating on a transmission medium by interleaving signal and pump wavelengths. In a preferred embodiment, the system comprises: (a) a pump for generating a plurality of pump wavelengths wherein at least one of the pump wavelengths is between two of the signal wavelengths; and (b) a coupler for coupling the pump wavelengths to the transmission medium such that the pump wavelengths and the signal wavelengths are counter-propagating. Preferably, the pump is adapted to generate pump wavelengths over a bandwidth greater than that of the peak Raman Stokes Shift.
Yet another aspect of the present invention is a long-haul cable system employing the amplification system as described above. In a preferred embodiment, the cable system comprises: (a) a transmission path; (b) a signal transmitter coupled to the transmission path and adapted for transmitting signal wavelengths; (c) a signal receiver coupled to the transmission path and adapted for receiving the signal wavelengths; and (d) at least one amplifier system as described above disposed along the transmission path between the signal transmitter and the signal receiver.